drm/amd/display: Add NULL pointer check for kzalloc

[Why & How]
Check return pointer of kzalloc before using it.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>